1. Field of the Invention
This invention relates generally to a high Tc superconducting Josephson junction element and, more particularly, to a superconducting Josephson junction element including a s-d wave junction formed only of a metal oxide superconductor.
2. Description of Prior Art
As a consequence of the development of high Tc superconductors, various superconducting devices operable at liquid nitrogen temperatures have been proposed. Such superconducting devices are expected to show much superior performance as compared with semiconductor devices and to be utilized as SQUIDs, digital devices and radiowave devices.
Superconductivity of known metal superconductors is able to be explained by the BCS theory. It is known that the superconductive energy gap of BCS superconductors is isotropic and has s-wave symmetry. In the case of metal oxide superconductors having a layered structure, on the other hand, there is suggested a possibility that the superconductive energy gap is anisotropic and has d-wave symmetry. However, symmetry of superconductive energy gap of metal oxide superconductors has not yet been clarified.
Recently, Iguchi et al and Wollman et al report that s-d-wave junctions have been formed between a conventional metal superconductor having s-wave symmetry and a metal oxide superconductor having a layered structure (I. Iguchi and Z. Wen, physical Review, vol. B49, p.12388 (1949); and D. A. Wollman, D. J. Van Harlingen, J. G. Giapintzakis and D. M. Ginsberg, physical Review Letters, vol. 74, p.797 (1995)). Namely, a junction element was prepared using a combination of pb having s-wave symmetry with YBa.sub.2 Cu.sub.3 O.sub.7-x (YBCO) film or YBCO single crystal having a layered structure. The junctions were subjected to various tests and were determined as being Josephson junctions.
More particularly, in Wollman et al, a YBCO/Au/pb junction is prepared. In view of the fact that (a) the junction shows current-voltage (I-V) pattern in the form of RSJ (resistivity shunt junction) model peculiar to SNS (superconductor/normal conductor/superconductor) junctions and (b) the maximum critical current flowing through the junction when a magnetic field is applied thereto shows a Fraunhofer-like pattern, the junction is determined as being a Josephson junction. Iguchi et al describe preperation of a YBCO/MgO/Pb junction. In view of the fact that (a) the junction shows a hysteresis in I-N characteristics that is peculiar to SIS (superconductor/insulator/superconductor) junctions and (b) the junction shows a Fraunhofer-like pattern against in a magnetic field, the junction is determined as being a Josephson junction.
In the junctions of Wollman et al and Iguchi et al, the Fraunhofer pattern differs from that of the conventional s-s-wave Josephson junctions using metal superconductors. Namely, whereas the superconductive critical current is maximum at the applied magnetic field of zero in the case of the Fraunhofer patterns of s-s-wave Josephson junctions, the junctions between pb and YBCO film and between pb and YBCO single crystal show minimum critical current at zero magnetic field. On the basis of these facts, it has been concluded that the junctions are s-d-wave junctions. The s-d-wave Josephson junctions are considered to provide superconducting devices having new functions different from the conventional s-s-wave junctions, because of the difference in Fraunhofer patterns therebetween.
In the case of the above s-d-wave Josephson junctions between the metal superconductor and YBCO, the operation temperature is determined by the critical temperature of the metal superconductor which is much lower than the liquid nitrogen temperature (77K). Thus, the known s-d-wave Josephson junctions cannot be integrated with other superconducting devices adapted to operate at the liquid nitrogen temperature.